Wireless communication devices are becoming increasingly popular, and as such, provide significant business opportunities to those with technologies that offer maximum performance and minimum costs. A successful wireless communication device provides clean, low noise signal transmission and reception at a reasonable cost and, in the case of portable devices, operates with low power consumption to maximize battery lifetime. A current industry focus is to monolithically integrate all the components needed for wireless communication onto one integrated circuit (IC) chip to further reduce the cost and size while enhancing performance.
One component of a wireless communication device that is not monolithically integrated on the IC is a switch. Switches are used for alternating between transmit and receive modes and are also used to switch filtering networks for channel discrimination. While solid state switches do exist and could possibly be integrated monolithically with other IC components, the moderate performance and relatively high cost of these switches has led to strong interest in micro electromechanical systems (MEMS) switches. MEMS switches are advantageously designed to operate with very low power consumption, offer equivalent if not superior performance, and can be monolithically integrated.
While MEMS switches have been under evaluation for several years, technical problems have delayed their immediate incorporation into wireless devices. One technical problem is the reliable actuation of the switch between the on and off states. This problem is exacerbated with the use of low switch actuation voltages, as is the case when these devices are integrated with advanced IC chips where available voltage signals are typically less than 10V. Prior art MEMS switch designs have been unable to provide reliable switching at low actuation voltages and power consumption while satisfying switch insertion loss and isolation specifications.
A typical design of a prior art MEMS switch is illustrated in FIGS. 1A-1B. MEMS switch 5 uses a pair of parallel electrodes 11 and 14 that are separated by a thin dielectric layer 12 and an air gap or cavity 13, bounded by dielectric standoffs 16. Electrode 14 is mounted on a membrane or movable beam which can be mechanically displaced. The other electrode 11 is bonded to substrate 10 and is not free to move. MEMS switch 5 has nominally two states, namely, open (as shown in FIG. 1A) or closed (as shown in FIG. 1B). In the open state, an air gap is present between electrodes 11 and 14 and the capacitance between these electrodes is low. In this state, an RF signal applied to electrode 14 would not be effectively coupled to electrode 11. MEMS switch 5 is closed by applying a DC electrostatic potential between the two electrodes 11 and 14, which displaces the movable electrode 14 to reduce the gap distance or make intimate contact with the dielectric layer 12 covering opposing electrode 11, as shown in FIG. 1B. Dielectric layer 12 prevents shorting the DC electrostatic potential between electrodes 11 and 14 and also defines the capacitance of the switch in the closed state. When electrode 14 contacts dielectric layer 12, the capacitance increases, and an RF signal on electrode 14 effectively couples to electrode 11. To deactivate the switch, the electrostatic potential is removed allowing the membrane (or beam) to mechanically return to its original position and restore gap 13 between the parallel electrodes. However, MEMS switch devices, by definition, are small, and effects such as dielectric charging and stiction often interfere with the reliable activation and deactivation of the MEMS switch. As noted above, for applications where MEMS switches are used in portable communication devices, the supply voltages allowed cannot reliably drive most prior art MEMS switches. For designs that insure reliable switch deactivation, unacceptably high voltages are required. Furthermore, these voltages must be increased over the lifetime of the switch due to a deterioration of the dielectric overcoat layer 12. For reliable switch activation, the membrane or movable beam is fabricated to have a low stiffness, which decreases the required actuation voltage and subsequent damage to dielectric overcoat 12. However, due to stiction, a low stiffness also increases the probability that the beam or membrane will not be deactivated when the activation voltage is removed, leaving the switch in the closed position. Moreover, MEMS switches used in portable communication devices also require low on insertion loss and high off-state isolation, which, in part, dictates the gap requirements between stationary electrode 11 and movable electrode 14.
To date, there is no known manufactured MEMS switch device that satisfies the reliability, low drive voltage, low power consumption, and signal attenuation requirements for portable communication device applications.